The invention concerns an apparatus for passively determining the thermal history of equipment by employing a combination of solid state track recorders ("SSTRs"). The apparatus is adapted to be used with a method disclosed in related application Ser. No. 07/481040, filed concurrently herewith, to determine an equivalent average temperature from which thermal aging of the equipment may be determined.
The properties of many materials undergo alteration as a result of exposure to temperature, for example, rubber gaskets may take a permanent set, flexible members may become brittle, organic fluids may decompose, electrical properties of insulating materials may vary, etc. This alteration in properties as a result of exposure to temperature is termed "thermal aging".
In nuclear power plants, many pieces of equipment, especially those concerned with the safety features of the plant, are "qualified." This qualification includes the determination of a qualified life, i.e. a maximum period of time for which the equipment may be placed in service. Due to the aforementioned thermal aging effect, the qualified life is a function of the temperature environment to which the equipment is subjected, as well as other factors, such as exposure to radiation. Some pieces of equipment may be comprised of various critical components each with its own qualified life. For maintenance purposes it is vital, therefore, to know when each piece of qualified equipment, or component therein, has reached its qualified life.
Theoretically, one could continuously measure and record the temperature to which each piece of qualified equipment was subjected (i.e. active temperature monitoring). This would allow the extent of thermal aging which the equipment had undergone, and therefore, the portion of its qualified life remaining, to be determined at any time. However, such an effort would be prohibitively costly, especially in certain areas of a nuclear power plant wherein high radiation levels make installing and maintaining temperature monitoring equipment difficult. A more common practice is to dispense with monitoring of the temperature environment by setting the qualified life conservatively low. This can be accomplished by assuming that the temperature is constantly maintained at the maximum foreseeable level for purposes of calculating the qualified life. However, this approach entails higher than necessary maintenance and equipment costs.
A second possible alternative to active monitoring would be to monitor the temperature environment of the equipment by passive means, thereby enabling the extent of thermal aging to be determined with less difficulty and expense. In a passive monitoring scheme, a passive temperature measuring device could be placed in close proximity to the equipment of interest so that the monitor would be subjected to the same temperature environment as the equipment. As a result of exposure to temperature, the device would undergo a detectable change (e.g., undergo thermal aging), the rate of this change being a known function of temperature. Typically, the higher the temperature the greater the rate of change. Since the extent of the change must be cumulative, such devices would be termed "integrating" thermal monitors. Since the period of time to which the device was exposed to the temperature environment would be known, and the temperature required to produce the observed change in such period of time is known, the temperature to which the device was exposed could be inferred by comparing the observed change to a calibration standard for the device. To date, no known suitable passive temperature monitor has been proposed.
One method for passive monitoring of a temperature environment, described in U.S. Pat. No. 4,167,109, involves the use of a solid state track recorder ("SSTR"). According to the patent, by determining the extent of annealing of the radiation "tracks" in the SSTR, the temperature to which the SSTR was exposed can be inferred. Unfortunately, use of SSTRs has been severely limited because no adequate means for relating extent of annealing to temperature exposure has heretofore been devised. As a result the use of SSTRs as devices for passive temperature monitoring, as described in Pat. No. 4,167,109, has never been developed. Their use has been confined to limited situations such as where the temperature of the environment is known to be essentially constant, so that the temperature history could be inferred from a single SSTR.
Consequently, the need exists for an apparatus and method for determining the thermal aging of equipment using passive monitoring means which can be utilized in variable temperature environments. This need is satisfied in the current invention by constructing a passive temperature monitoring device comprised of a plurality of SSTRs. The data from such device can be analyzed using Arrhenius functions to obtain an equivalent average temperature. This equivalent average temperature is representative of the thermal history of the equipment and can be combined with thermal aging data for the equipment to determine its useful life.
The Arrhenius function has been used in the past to analyze the results of accelerated thermal aging tests, for example as disclosed in S. Carfagno and R. Gibson, A Review of Equipment Aging Theory and Technology, EPRI Equipment Aging Theory and Technologyov, EPRI Report, NP-1558, .sctn.8.3 (1980), and to analyze data from naturally occurring particle tracks in naturally occurring glasses for geological dating purposes, for example as disclosed in D. Storzer, "Fission Track Dating of Volcanic Glasses and the Thermal History of Rocks," in Earth and Planetary Science Letters, 8, pp 55-60 (1970. However, its use as described herein, to determine an equivalent average temperature obtained from passive devices which can be related to the thermal aging of the equipment being monitored, is believed to be new.